The overall goal of our translational research program is to understand the interplay between activation of the immune system and mitochondrial metabolism (i.e. immunometabolism) using mitochondrial disease as a model system. Over this past year we have made considerable progress in our program. Last summer we published a paper demonstrating the role of cytochrome c oxidase (COX, complex IV in OXPHOS) in T-cell activation (Cell Metab, 2017). We began by examining patients with various OXPHOS deficiencies via the NIH MINI study. Recognizing an immune phenotype involving T-cell memory responses, we constructed a model of T-cell COX deficiency by targeting COX10, an assembly factor for COX. Using this model, we demonstrated the unique role of COX as a metabolic checkpoint in T-cell activation by mediating apoptosis. Following up on this publication, we created a mouse model of SURF1 deficiency using CRISPR. SURF1 is another COX assembly factor that produces severe mitochondrial disease in humans. To our surprise, despite having low COX activity, this mouse model displays normal T-cell function, unlike COX10 deficient mice. Both models will allow us to dissect COX functional domains and their role in T-cell function. Furthermore, we will also explore therapies aimed at bypassing COX deficiency in COX10 mice by expressing an alternative oxidase (AOX) in T-cells. Continuing on this theme of the role of mitochondrial metabolism in T-cells, we have been involved in an international collaborative project addressing a fundamental question regarding the role of mitochondrial fatty acid oxidation in T-cells. The accepted dogma is that mitochondrial fatty acid oxidation promotes memory T-cell formation. Using patient clinical data and mouse models of fatty acid oxidation, we aided in demonstrating that mitochondrial fatty acid oxidation was dispensable for determining T-cell memory phenotypes. As s result, we contributed to a seminal publication (Cell Metab, 2018) and high profile review (Immunol Rev 2018) challenging this dogma. Both of the aforementioned accomplishments highlight the central role clinical research plays in our program. Based on our patient-centered approach to addressing fundamental questions in immunometabolism, we published a review extolling the virtues of studying mitochondrial disease as a model system for answering critical questions regarding the role of the mitochondria in immune cell function (Metabolism 2018). In addition to work in T-cell immunometabolism, we have also continued to explore the role of the immune system in modulating end organ metabolism. Using a metabolomics approach with complex data reduction techniques, we characterized the pathophysiology of metabolic perturbations that occur as a result of infection in a mouse model of mitochondrial long chain fatty acid oxidation (Mol Genet Metab, 2018). This paper and a review on the role of the immune system in promoting metabolic dysregulation in the liver during infection (Mol Genet Metab, 2017) highlights the role of the innate immune system and resident macrophages (e.g. Kupffer cells) in modulating end organ metabolism. This work continues in a mouse model of mitochondrial hepatopathy where targeting the immune system is being explored as a therapeutic avenue.